Android BitmapFactory decodeFile 在它应该被调用之前被调用

Question

我正在尝试在 ListView 中显示图像的缩略图。这些图像位于我的 SDCard 上的 Camera 文件夹中。我正在使用 BitmapFactory.decodeFile 读取图像。我想在文件被解码时显示一个 ProgressDialog。我试图先显示 ProgressDialog，然后在 for 循环中调用 decodeFile。直到 for 循环之后才会显示 ProgressDialog。在显示 ProgressDialog 之前，对 decodeFile 的调用似乎正在运行。

如何在我的 for 循环之前显示 ProgressDialog？

公共类 ActivityProgressBar 扩展 ListActivity {

private Vector<RowFileData> fileDataDisplay = null;     
RowFileData fileData;
ProgressDialog progressDialog = null;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.list);        
    progressDialog = new ProgressDialog(this);
    progressDialog.setMessage("Loading thumbnails...");
    fileDataDisplay = new Vector<RowFileData>();
    File currentDirectory = new File("/mnt/sdcard/dcim/camera");
    File[] currentDirectoryFileList = currentDirectory.listFiles();
    BitmapFactory.Options opts = new BitmapFactory.Options();
    opts.inSampleSize = 16;
    progressDialog.show();
    for(int i=0; i<currentDirectoryFileList.length; i++)
    {
        File currentDirectoryFile = currentDirectoryFileList[i];
        fileData = new RowFileData(BitmapFactory.decodeFile(currentDirectoryFile.getPath(), opts), currentDirectoryFile.getPath());
        fileDataDisplay.add(fileData);
        Log.v("myLog", "inside for loop");
    }
}   

private class RowFileData 
{
   protected Bitmap rowBitmap;
   protected String rowFileName;
   RowFileData(Bitmap bitmapPreview, String fileName)
   {
       rowBitmap = bitmapPreview;
       rowFileName = fileName;
   }
}

}

我已经注释掉了对 progressDialog.dismiss() 的调用，以验证 ProgressDialog 是否在 for 循环之后显示。为了便于阅读，我删除了在 ListView 中显示图像的代码行。我用我的日志验证了我仍在 for 循环中。

谢谢

Answer 1

您还可以使用 asynctask 在后台解码位图并在前面显示进度对话框。完成解码位图后，只需禁用进度对话框。 Android - AsyncTask 和 Tutorial 帮助你。谢谢。

Answer 2

我建议查看延迟加载您的图像。在另一篇 SO 帖子中有一个很好的示例，我刚刚为此目的实现了它，并且效果很好。

Lazy load of images in ListView

Answer 3

我希望在解码位图文件时显示进度对话框并保持显示。另外，我没有意识到Android中的主线程是UI线程，并且在主线程上进行处理时UI被冻结！我使用了后台线程，现在进度对话框正确显示：

公共类 ActivityProgressBar 扩展 ListActivity {

private Vector<RowFileData> fileDataDisplay = null;     
RowFileData fileData;
ProgressDialog progressDialog = null;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.list);        
    progressDialog = new ProgressDialog(this);
    progressDialog.setMessage("Loading thumbnails...");
    fileDataDisplay = new Vector<RowFileData>();
    File currentDirectory = new File("/mnt/sdcard/dcim/camera");
    BitmapFactory.Options opts = new BitmapFactory.Options();
    opts.inSampleSize = 16;
    File[] currentDirectoryFileList = currentDirectory.listFiles();
    progressDialog.show();
    readThumbnails(currentDirectoryFileList, opts);
}   

private void readThumbnails(final File[] currentDirectoryFileList, final BitmapFactory.Options opts) 
{
    Thread backgroundThread = new Thread()
    {
        public void run()
        {
            try
            {
                for(int i=0; i<currentDirectoryFileList.length; i++)
                {
                    File currentDirectoryFile = currentDirectoryFileList[i];
                    fileData = new RowFileData(BitmapFactory.decodeFile(currentDirectoryFile.getPath(), opts), currentDirectoryFile.getPath());
                    fileDataDisplay.add(fileData);
                    Log.v("myLog", "inside for loop");
                }
                uiCallback.sendEmptyMessage(0);
            }
            catch(Exception ex)
            {
                Log.v("myLog", ex.toString());
            }
        }
    };
    backgroundThread.start();
}

private Handler uiCallback = new Handler()
{
    @Override
    public void handleMessage(Message emptyMessage)
    {
        progressDialog.dismiss();
    }
};

private class RowFileData 
{
   protected Bitmap rowBitmap;
   protected String rowFileName;
   RowFileData(Bitmap bitmapPreview, String fileName)
   {
       rowBitmap = bitmapPreview;
       rowFileName = fileName;
   }
}

}